1. Field of the Invention
The present invention relates to filler materials for adding to various matrix materials, and capacitors, especially for making electrodes for electric double-layer capacitors, secondary batteries, especially lithium- and lead-based secondary batteries.
2. Description of the Related Art
Carbon fibers produced in vapor phase (vapor grown carbon fibers) are added as a filler agent to various polymeric materials, organometallic materials as well as metals and ceramics to improve their properties such as electrical conductivity, frictional characteristics, thermal conductivity and mechanical strength.
In general, vapor grown carbon fibers (VGCF) are available in dimensions of 0.05.about.5 .mu.m diameter and 1.about.1,000 .mu.m length, and their features include well developed graphitic cell surfaces along the fiber axis and hollow interior. In the as-grown condition, these micro-fibers assume a cluster form of a low apparent density.
In general, carbonaceous substances such as tar and pitch remain in such fiber clusters, which can be carbonized by heating and such converted materials are removed as necessary. This heating process improves the properties of the carbon fibers thus produced.
Specific heating procedures for such vapor grown carbon fibers include, for example, a method based on continuous or intermittent heating of carbon fibers in a heat resistant container (Japanese Patent Application, First Publication (A), H1-272827); a method based on kneading fiber clusters and heating (Japanese Patent Application, First Publication (A), H1-270543); and a method based on heating the carbon fibers molded into certain shapes (Japanese Patent Application, First Publication (A), H1-290570).
When the carbon fiber clusters are to be used as a filler in high molecular weight materials, it is necessary that the fiber clusters be ground to a powder. A specific example includes grinding the carbon fiber clusters minutely with the use of a jet mill (Japanese Patent Application, First Publication (A), S63-21208 and Japanese Patent Application, First Publication (A), S63-283766), and highspeed rolling in a grinding mixture of balls of less than 10 mm diameter, made of ceramics such as zirconia and alumina and hard metals (Japanese Patent Application, First Publication (A), H6-32607). A method based on grinding the singular fibers themselves is also disclosed (Japanese Patent Application, First Publication (A), H4-222227).
Although the vapor grown carbon fibers themselves exhibit good electrical and thermal conductivity properties, the present state of the art is such that when they are used as a filler material in making polymeric composite products, their favorable electrical and thermal conductive are not reflected in the properties of the composite product made therewith.
In considering a case of improving the electrical conductivity of a composite product, the filler should be generally in a form of fibers because it is believed that the conductivity paths should be made long.
However, the longer the fibers the higher the tendency for the fibers to orient in a preferred direction, and a resin mixture having such a fiber alignment depresses the fluidity and consequently the formability of the composite feed.
The result is that the present practice is to use whisker fibers that are microscopic such as those as-grown fibers or milled singular fibers made therefrom. Especially, it is considered that preferred fibers are singular fibers that have aspect ratios of greater than 10 preferably more than 100 and have the length dimensions in the range of several tens of microns.
However, as the fibers become smaller, the number of contact points of the fibers also increases and the electrical contact resistance is increased. Furthermore, contact points of the fibers are vulnerable to infiltration by the matrix resin during molding, and the electrical conductivity of the composite product is ultimately degraded.
Regarding the method of heating the carbon fibers in a heat resistant container or the method based on kneading fiber clusters and heating mentioned above, the difficulty has been that the density of the fiber cluster is low because of insufficient pressure, so that not only the fiber contact points are few but bonding at the fiber contact points is insufficient.
Regarding the method of grinding the fibers minutely, the difficulty has been that, because the fibers are milled too fine, too many fiber contact points are created, which increase the electrical contact resistance. When such a composite feed is formed into a product, the fiber contact points are broken by the infiltrated resin during the molding process, resulting that the electrical conductivity cannot be increased.
The above Japanese Patent Application, First Publication (A), H1-290570 discloses a method of forming and sintering of the fibers, but the fibers are meant to be used for thermal insulation, and are not for use as a filler material.
Therefore, it can be understood that the VGCF at the present time are not able to demonstrate their full potential in improving the electrical and thermal conductivity characteristics of composite materials.
Fibrous fillers themselves exhibit an effective electrical conductivity in two-dimensions because of their lengthwise geometry, but, in a composite material, it is required that the electrical conductivity property be improved in three-dimensions. It follows that the fibrous material for making a composite product should have a structure such that the properties are isotropic in three-dimensions and that the conductivity path be as long as possible. It may be considered that, to simply increase the path lengths in three-dimensions, larger particles would be preferable. However, large particles are not suitable as a filler material because, when added to a matrix, such large particles not only diminish the contact point per unit volume of the matrix but adversely affect the formability and strength properties of the composite product.
Therefore, to increase the electrical conductivity in a composite product while retaining the strength and formability properties of the composite material, it is desirable to have a fibrous structure that creates a functional three-dimensional structure with a high number of contact points which retains the integrity of the electrical conductive paths.
Such a filler material comprised by vapor grown carbon fibers and has an ideal fiber structure is not available currently.
In recent years, there have been developments and commercialization of charge storage devices based on electric double-layers, that is, electric double-layer capacitors derived from the electric double-layer principle. Because of high static capacitance achievable in electric double-layer capacitors, in small size capacitors are suitable as backup electrical source for semiconductor memories in electronic apparatus while large devices are even starting to be used as a part of lead storage batteries for vehicular power applications.
Generally speaking, carbon electrodes are used in electric double-layer capacitors. These electric double-layer capacitors utilize electric double-layers formed near the interfaces between the electrode surface and electrolyte, therefore, it is necessary to increase the surface area of the carbon electrodes.
For this reason, various electrode designs have been proposed such as, for example, carbon electrodes made of activated charcoal particles having micro-porosities (Japanese Patent Application, First Publication (A), S63-18761: and Japanese Patent Application, First Publication (A), H1-227417); carbon electrodes produced by heating a polyvinylidene chloride resin in a non-oxidizing atmosphere so as to generate atomic and molecular faults to form micro-porosities (Japanese Patent Application, First Publication (A), H7-249551).
On the other hand, electrode plates of high capacitance require not only high electrode surface area (i.e. specific surface area) but also high electrical and thermal conductivities.
However, the activated charcoal particles mentioned above have low crystallinity and the electrical resistance of the material itself is high, and it is difficult to obtain sufficient electrical conductivity from the electrodes made from such activated charcoal particles because of the high inter-particle electrical contact resistance.
For this reason, there has been a proposal for an electric double-layer capacitor which used electrodes having activated charcoal particles and carbon whiskers (Japanese Patent Application, First Publication (A), H7-307250).
However, although a filler material containing singular form of fiber carbon whiskers provides two-dimensional conductance effects, as discussed above, it does not provide sufficient conductance in three-dimensions. It is better and preferable that the electrode plates for electric double-layer capacitor be uniform in their overall three-dimensional properties.
To improve the electrical conductivity, a filler material should have a structure exhibiting isotropic properties in three dimensions and provide as long a conductive distance as possible, and additionally, it is preferable that the filler material should have a web-like structure that enables to support sufficient quantity of activated charcoal particles.
However, at the present time, there is no electrode material to fulfill such requirements for making high performance electric double-layer capacitors.
Some secondary batteries, in particular lithium- or lead-based secondary batteries, use electrodes containing carbon fibers, especially VGCF.
For example, negative electrodes, in lithium ion secondary batteries, made from a mixture of VGCF and graphite powder obtained by high temperature processing of cokes are known (Japanese Patent Application, First Publication (A), H4-155776).
Also, negative electrodes in lithium ion secondary batteries made from a mixture of meso-carbon microbeads and VGCF are known (Japanese Patent Application, First Publication (A), H4-237971).
These lithium secondary batteries are said to prevent expansion of the electrode active material such as graphite powder, thus suppressing grid deformation and fracture generated during repeated charge/discharge cycles, thereby resisting degradation of their charge storage capacity.
However, in such conventional electrode materials, because the graphite powder and like others and VGCF exist simply in a mechanically blended form, the carbon fibers are not able to sufficiently suppress destruction of the electrodes by grid expansion, and it is insufficient to prevent the consequent loss of capacity degradation.